compiler/rustc_hir_typeck/src/demand.rs

   1 use crate::FnCtxt;
   2 use rustc_ast::util::parser::PREC_POSTFIX;
   3 use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed};
   4 use rustc_hir as hir;
   5 use rustc_hir::lang_items::LangItem;
   6 use rustc_hir::{is_range_literal, Node};
   7 use rustc_infer::infer::InferOk;
   8 use rustc_middle::lint::in_external_macro;
   9 use rustc_middle::middle::stability::EvalResult;
  10 use rustc_middle::ty::adjustment::AllowTwoPhase;
  11 use rustc_middle::ty::error::{ExpectedFound, TypeError};
  12 use rustc_middle::ty::print::with_no_trimmed_paths;
  13 use rustc_middle::ty::{self, Article, AssocItem, Ty, TypeAndMut};
  14 use rustc_span::symbol::{sym, Symbol};
  15 use rustc_span::{BytePos, Span};
  16 use rustc_trait_selection::infer::InferCtxtExt as _;
  17 use rustc_trait_selection::traits::ObligationCause;
  18
  19 use super::method::probe;
  20
  21 use std::iter;
  22
  23 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  24     pub fn emit_coerce_suggestions(
  25         &self,
  26         err: &mut Diagnostic,
  27         expr: &hir::Expr<'tcx>,
  28         expr_ty: Ty<'tcx>,
  29         expected: Ty<'tcx>,
  30         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
  31         error: Option<TypeError<'tcx>>,
  32     ) {
  33         if expr_ty == expected {
  34             return;
  35         }
  36
  37         self.annotate_expected_due_to_let_ty(err, expr, error);
  38
  39         // Use `||` to give these suggestions a precedence
  40         let _ = self.suggest_missing_parentheses(err, expr)
  41             || self.suggest_deref_ref_or_into(err, expr, expected, expr_ty, expected_ty_expr)
  42             || self.suggest_compatible_variants(err, expr, expected, expr_ty)
  43             || self.suggest_non_zero_new_unwrap(err, expr, expected, expr_ty)
  44             || self.suggest_calling_boxed_future_when_appropriate(err, expr, expected, expr_ty)
  45             || self.suggest_no_capture_closure(err, expected, expr_ty)
  46             || self.suggest_boxing_when_appropriate(err, expr, expected, expr_ty)
  47             || self.suggest_block_to_brackets_peeling_refs(err, expr, expr_ty, expected)
  48             || self.suggest_copied_or_cloned(err, expr, expr_ty, expected)
  49             || self.suggest_into(err, expr, expr_ty, expected)
  50             || self.suggest_option_to_bool(err, expr, expr_ty, expected);
  51
  52         self.note_type_is_not_clone(err, expected, expr_ty, expr);
  53         self.note_need_for_fn_pointer(err, expected, expr_ty);
  54         self.note_internal_mutation_in_method(err, expr, expected, expr_ty);
  55     }
  56
  57     // Requires that the two types unify, and prints an error message if
  58     // they don't.
  59     pub fn demand_suptype(&self, sp: Span, expected: Ty<'tcx>, actual: Ty<'tcx>) {
  60         if let Some(mut e) = self.demand_suptype_diag(sp, expected, actual) {
  61             e.emit();
  62         }
  63     }
  64
  65     pub fn demand_suptype_diag(
  66         &self,
  67         sp: Span,
  68         expected: Ty<'tcx>,
  69         actual: Ty<'tcx>,
  70     ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
  71         self.demand_suptype_with_origin(&self.misc(sp), expected, actual)
  72     }
  73
  74     #[instrument(skip(self), level = "debug")]
  75     pub fn demand_suptype_with_origin(
  76         &self,
  77         cause: &ObligationCause<'tcx>,
  78         expected: Ty<'tcx>,
  79         actual: Ty<'tcx>,
  80     ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
  81         match self.at(cause, self.param_env).sup(expected, actual) {
  82             Ok(InferOk { obligations, value: () }) => {
  83                 self.register_predicates(obligations);
  84                 None
  85             }
  86             Err(e) => Some(self.err_ctxt().report_mismatched_types(&cause, expected, actual, e)),
  87         }
  88     }
  89
  90     pub fn demand_eqtype(&self, sp: Span, expected: Ty<'tcx>, actual: Ty<'tcx>) {
  91         if let Some(mut err) = self.demand_eqtype_diag(sp, expected, actual) {
  92             err.emit();
  93         }
  94     }
  95
  96     pub fn demand_eqtype_diag(
  97         &self,
  98         sp: Span,
  99         expected: Ty<'tcx>,
 100         actual: Ty<'tcx>,
 101     ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
 102         self.demand_eqtype_with_origin(&self.misc(sp), expected, actual)
 103     }
 104
 105     pub fn demand_eqtype_with_origin(
 106         &self,
 107         cause: &ObligationCause<'tcx>,
 108         expected: Ty<'tcx>,
 109         actual: Ty<'tcx>,
 110     ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
 111         match self.at(cause, self.param_env).eq(expected, actual) {
 112             Ok(InferOk { obligations, value: () }) => {
 113                 self.register_predicates(obligations);
 114                 None
 115             }
 116             Err(e) => Some(self.err_ctxt().report_mismatched_types(cause, expected, actual, e)),
 117         }
 118     }
 119
 120     pub fn demand_coerce(
 121         &self,
 122         expr: &hir::Expr<'tcx>,
 123         checked_ty: Ty<'tcx>,
 124         expected: Ty<'tcx>,
 125         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
 126         allow_two_phase: AllowTwoPhase,
 127     ) -> Ty<'tcx> {
 128         let (ty, err) =
 129             self.demand_coerce_diag(expr, checked_ty, expected, expected_ty_expr, allow_two_phase);
 130         if let Some(mut err) = err {
 131             err.emit();
 132         }
 133         ty
 134     }
 135
 136     /// Checks that the type of `expr` can be coerced to `expected`.
 137     ///
 138     /// N.B., this code relies on `self.diverges` to be accurate. In particular, assignments to `!`
 139     /// will be permitted if the diverges flag is currently "always".
 140     #[instrument(level = "debug", skip(self, expr, expected_ty_expr, allow_two_phase))]
 141     pub fn demand_coerce_diag(
 142         &self,
 143         expr: &hir::Expr<'tcx>,
 144         checked_ty: Ty<'tcx>,
 145         expected: Ty<'tcx>,
 146         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
 147         allow_two_phase: AllowTwoPhase,
 148     ) -> (Ty<'tcx>, Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>>) {
 149         let expected = self.resolve_vars_with_obligations(expected);
 150
 151         let e = match self.try_coerce(expr, checked_ty, expected, allow_two_phase, None) {
 152             Ok(ty) => return (ty, None),
 153             Err(e) => e,
 154         };
 155
 156         self.set_tainted_by_errors();
 157         let expr = expr.peel_drop_temps();
 158         let cause = self.misc(expr.span);
 159         let expr_ty = self.resolve_vars_with_obligations(checked_ty);
 160         let mut err = self.err_ctxt().report_mismatched_types(&cause, expected, expr_ty, e.clone());
 161
 162         let is_insufficiently_polymorphic =
 163             matches!(e, TypeError::RegionsInsufficientlyPolymorphic(..));
 164
 165         // FIXME(#73154): For now, we do leak check when coercing function
 166         // pointers in typeck, instead of only during borrowck. This can lead
 167         // to these `RegionsInsufficientlyPolymorphic` errors that aren't helpful.
 168         if !is_insufficiently_polymorphic {
 169             self.emit_coerce_suggestions(
 170                 &mut err,
 171                 expr,
 172                 expr_ty,
 173                 expected,
 174                 expected_ty_expr,
 175                 Some(e),
 176             );
 177         }
 178
 179         (expected, Some(err))
 180     }
 181
 182     fn annotate_expected_due_to_let_ty(
 183         &self,
 184         err: &mut Diagnostic,
 185         expr: &hir::Expr<'_>,
 186         error: Option<TypeError<'_>>,
 187     ) {
 188         let parent = self.tcx.hir().get_parent_node(expr.hir_id);
 189         match (self.tcx.hir().find(parent), error) {
 190             (Some(hir::Node::Local(hir::Local { ty: Some(ty), init: Some(init), .. })), _)
 191                 if init.hir_id == expr.hir_id =>
 192             {
 193                 // Point at `let` assignment type.
 194                 err.span_label(ty.span, "expected due to this");
 195             }
 196             (
 197                 Some(hir::Node::Expr(hir::Expr {
 198                     kind: hir::ExprKind::Assign(lhs, rhs, _), ..
 199                 })),
 200                 Some(TypeError::Sorts(ExpectedFound { expected, .. })),
 201             ) if rhs.hir_id == expr.hir_id && !expected.is_closure() => {
 202                 // We ignore closures explicitly because we already point at them elsewhere.
 203                 // Point at the assigned-to binding.
 204                 let mut primary_span = lhs.span;
 205                 let mut secondary_span = lhs.span;
 206                 let mut post_message = "";
 207                 match lhs.kind {
 208                     hir::ExprKind::Path(hir::QPath::Resolved(
 209                         None,
 210                         hir::Path {
 211                             res:
 212                                 hir::def::Res::Def(
 213                                     hir::def::DefKind::Static(_) | hir::def::DefKind::Const,
 214                                     def_id,
 215                                 ),
 216                             ..
 217                         },
 218                     )) => {
 219                         if let Some(hir::Node::Item(hir::Item {
 220                             ident,
 221                             kind: hir::ItemKind::Static(ty, ..) | hir::ItemKind::Const(ty, ..),
 222                             ..
 223                         })) = self.tcx.hir().get_if_local(*def_id)
 224                         {
 225                             primary_span = ty.span;
 226                             secondary_span = ident.span;
 227                             post_message = " type";
 228                         }
 229                     }
 230                     hir::ExprKind::Path(hir::QPath::Resolved(
 231                         None,
 232                         hir::Path { res: hir::def::Res::Local(hir_id), .. },
 233                     )) => {
 234                         if let Some(hir::Node::Pat(pat)) = self.tcx.hir().find(*hir_id) {
 235                             let parent = self.tcx.hir().get_parent_node(pat.hir_id);
 236                             primary_span = pat.span;
 237                             secondary_span = pat.span;
 238                             match self.tcx.hir().find(parent) {
 239                                 Some(hir::Node::Local(hir::Local { ty: Some(ty), .. })) => {
 240                                     primary_span = ty.span;
 241                                     post_message = " type";
 242                                 }
 243                                 Some(hir::Node::Local(hir::Local { init: Some(init), .. })) => {
 244                                     primary_span = init.span;
 245                                     post_message = " value";
 246                                 }
 247                                 Some(hir::Node::Param(hir::Param { ty_span, .. })) => {
 248                                     primary_span = *ty_span;
 249                                     post_message = " parameter type";
 250                                 }
 251                                 _ => {}
 252                             }
 253                         }
 254                     }
 255                     _ => {}
 256                 }
 257
 258                 if primary_span != secondary_span
 259                     && self
 260                         .tcx
 261                         .sess
 262                         .source_map()
 263                         .is_multiline(secondary_span.shrink_to_hi().until(primary_span))
 264                 {
 265                     // We are pointing at the binding's type or initializer value, but it's pattern
 266                     // is in a different line, so we point at both.
 267                     err.span_label(secondary_span, "expected due to the type of this binding");
 268                     err.span_label(primary_span, &format!("expected due to this{post_message}"));
 269                 } else if post_message == "" {
 270                     // We are pointing at either the assignment lhs or the binding def pattern.
 271                     err.span_label(primary_span, "expected due to the type of this binding");
 272                 } else {
 273                     // We are pointing at the binding's type or initializer value.
 274                     err.span_label(primary_span, &format!("expected due to this{post_message}"));
 275                 }
 276
 277                 if !lhs.is_syntactic_place_expr() {
 278                     // We already emitted E0070 "invalid left-hand side of assignment", so we
 279                     // silence this.
 280                     err.downgrade_to_delayed_bug();
 281                 }
 282             }
 283             _ => {}
 284         }
 285     }
 286
 287     /// If the expected type is an enum (Issue #55250) with any variants whose
 288     /// sole field is of the found type, suggest such variants. (Issue #42764)
 289     fn suggest_compatible_variants(
 290         &self,
 291         err: &mut Diagnostic,
 292         expr: &hir::Expr<'_>,
 293         expected: Ty<'tcx>,
 294         expr_ty: Ty<'tcx>,
 295     ) -> bool {
 296         if let ty::Adt(expected_adt, substs) = expected.kind() {
 297             if let hir::ExprKind::Field(base, ident) = expr.kind {
 298                 let base_ty = self.typeck_results.borrow().expr_ty(base);
 299                 if self.can_eq(self.param_env, base_ty, expected).is_ok()
 300                     && let Some(base_span) = base.span.find_ancestor_inside(expr.span)
 301                 {
 302                     err.span_suggestion_verbose(
 303                         expr.span.with_lo(base_span.hi()),
 304                         format!("consider removing the tuple struct field `{ident}`"),
 305                         "",
 306                         Applicability::MaybeIncorrect,
 307                     );
 308                     return true;
 309                 }
 310             }
 311
 312             // If the expression is of type () and it's the return expression of a block,
 313             // we suggest adding a separate return expression instead.
 314             // (To avoid things like suggesting `Ok(while .. { .. })`.)
 315             if expr_ty.is_unit() {
 316                 let mut id = expr.hir_id;
 317                 let mut parent;
 318
 319                 // Unroll desugaring, to make sure this works for `for` loops etc.
 320                 loop {
 321                     parent = self.tcx.hir().get_parent_node(id);
 322                     if let Some(parent_span) = self.tcx.hir().opt_span(parent) {
 323                         if parent_span.find_ancestor_inside(expr.span).is_some() {
 324                             // The parent node is part of the same span, so is the result of the
 325                             // same expansion/desugaring and not the 'real' parent node.
 326                             id = parent;
 327                             continue;
 328                         }
 329                     }
 330                     break;
 331                 }
 332
 333                 if let Some(hir::Node::Block(&hir::Block {
 334                     span: block_span, expr: Some(e), ..
 335                 })) = self.tcx.hir().find(parent)
 336                 {
 337                     if e.hir_id == id {
 338                         if let Some(span) = expr.span.find_ancestor_inside(block_span) {
 339                             let return_suggestions = if self
 340                                 .tcx
 341                                 .is_diagnostic_item(sym::Result, expected_adt.did())
 342                             {
 343                                 vec!["Ok(())"]
 344                             } else if self.tcx.is_diagnostic_item(sym::Option, expected_adt.did()) {
 345                                 vec!["None", "Some(())"]
 346                             } else {
 347                                 return false;
 348                             };
 349                             if let Some(indent) =
 350                                 self.tcx.sess.source_map().indentation_before(span.shrink_to_lo())
 351                             {
 352                                 // Add a semicolon, except after `}`.
 353                                 let semicolon =
 354                                     match self.tcx.sess.source_map().span_to_snippet(span) {
 355                                         Ok(s) if s.ends_with('}') => "",
 356                                         _ => ";",
 357                                     };
 358                                 err.span_suggestions(
 359                                     span.shrink_to_hi(),
 360                                     "try adding an expression at the end of the block",
 361                                     return_suggestions
 362                                         .into_iter()
 363                                         .map(|r| format!("{semicolon}\n{indent}{r}")),
 364                                     Applicability::MaybeIncorrect,
 365                                 );
 366                             }
 367                             return true;
 368                         }
 369                     }
 370                 }
 371             }
 372
 373             let compatible_variants: Vec<(String, _, _, Option<String>)> = expected_adt
 374                 .variants()
 375                 .iter()
 376                 .filter(|variant| {
 377                     variant.fields.len() == 1
 378                 })
 379                 .filter_map(|variant| {
 380                     let sole_field = &variant.fields[0];
 381
 382                     let field_is_local = sole_field.did.is_local();
 383                     let field_is_accessible =
 384                         sole_field.vis.is_accessible_from(expr.hir_id.owner.def_id, self.tcx)
 385                         // Skip suggestions for unstable public fields (for example `Pin::pointer`)
 386                         && matches!(self.tcx.eval_stability(sole_field.did, None, expr.span, None), EvalResult::Allow | EvalResult::Unmarked);
 387
 388                     if !field_is_local && !field_is_accessible {
 389                         return None;
 390                     }
 391
 392                     let note_about_variant_field_privacy = (field_is_local && !field_is_accessible)
 393                         .then(|| format!(" (its field is private, but it's local to this crate and its privacy can be changed)"));
 394
 395                     let sole_field_ty = sole_field.ty(self.tcx, substs);
 396                     if self.can_coerce(expr_ty, sole_field_ty) {
 397                         let variant_path =
 398                             with_no_trimmed_paths!(self.tcx.def_path_str(variant.def_id));
 399                         // FIXME #56861: DRYer prelude filtering
 400                         if let Some(path) = variant_path.strip_prefix("std::prelude::")
 401                             && let Some((_, path)) = path.split_once("::")
 402                         {
 403                             return Some((path.to_string(), variant.ctor_kind, sole_field.name, note_about_variant_field_privacy));
 404                         }
 405                         Some((variant_path, variant.ctor_kind, sole_field.name, note_about_variant_field_privacy))
 406                     } else {
 407                         None
 408                     }
 409                 })
 410                 .collect();
 411
 412             let suggestions_for = |variant: &_, ctor, field_name| {
 413                 let prefix = match self.maybe_get_struct_pattern_shorthand_field(expr) {
 414                     Some(ident) => format!("{ident}: "),
 415                     None => String::new(),
 416                 };
 417
 418                 let (open, close) = match ctor {
 419                     hir::def::CtorKind::Fn => ("(".to_owned(), ")"),
 420                     hir::def::CtorKind::Fictive => (format!(" {{ {field_name}: "), " }"),
 421
 422                     // unit variants don't have fields
 423                     hir::def::CtorKind::Const => unreachable!(),
 424                 };
 425
 426                 // Suggest constructor as deep into the block tree as possible.
 427                 // This fixes https://github.com/rust-lang/rust/issues/101065,
 428                 // and also just helps make the most minimal suggestions.
 429                 let mut expr = expr;
 430                 while let hir::ExprKind::Block(block, _) = &expr.kind
 431                     && let Some(expr_) = &block.expr
 432                 {
 433                     expr = expr_
 434                 }
 435
 436                 vec![
 437                     (expr.span.shrink_to_lo(), format!("{prefix}{variant}{open}")),
 438                     (expr.span.shrink_to_hi(), close.to_owned()),
 439                 ]
 440             };
 441
 442             match &compatible_variants[..] {
 443                 [] => { /* No variants to format */ }
 444                 [(variant, ctor_kind, field_name, note)] => {
 445                     // Just a single matching variant.
 446                     err.multipart_suggestion_verbose(
 447                         &format!(
 448                             "try wrapping the expression in `{variant}`{note}",
 449                             note = note.as_deref().unwrap_or("")
 450                         ),
 451                         suggestions_for(&**variant, *ctor_kind, *field_name),
 452                         Applicability::MaybeIncorrect,
 453                     );
 454                     return true;
 455                 }
 456                 _ => {
 457                     // More than one matching variant.
 458                     err.multipart_suggestions(
 459                         &format!(
 460                             "try wrapping the expression in a variant of `{}`",
 461                             self.tcx.def_path_str(expected_adt.did())
 462                         ),
 463                         compatible_variants.into_iter().map(
 464                             |(variant, ctor_kind, field_name, _)| {
 465                                 suggestions_for(&variant, ctor_kind, field_name)
 466                             },
 467                         ),
 468                         Applicability::MaybeIncorrect,
 469                     );
 470                     return true;
 471                 }
 472             }
 473         }
 474
 475         false
 476     }
 477
 478     fn suggest_non_zero_new_unwrap(
 479         &self,
 480         err: &mut Diagnostic,
 481         expr: &hir::Expr<'_>,
 482         expected: Ty<'tcx>,
 483         expr_ty: Ty<'tcx>,
 484     ) -> bool {
 485         let tcx = self.tcx;
 486         let (adt, unwrap) = match expected.kind() {
 487             // In case Option<NonZero*> is wanted, but * is provided, suggest calling new
 488             ty::Adt(adt, substs) if tcx.is_diagnostic_item(sym::Option, adt.did()) => {
 489                 // Unwrap option
 490                 let ty::Adt(adt, _) = substs.type_at(0).kind() else { return false; };
 491
 492                 (adt, "")
 493             }
 494             // In case NonZero* is wanted, but * is provided also add `.unwrap()` to satisfy types
 495             ty::Adt(adt, _) => (adt, ".unwrap()"),
 496             _ => return false,
 497         };
 498
 499         let map = [
 500             (sym::NonZeroU8, tcx.types.u8),
 501             (sym::NonZeroU16, tcx.types.u16),
 502             (sym::NonZeroU32, tcx.types.u32),
 503             (sym::NonZeroU64, tcx.types.u64),
 504             (sym::NonZeroU128, tcx.types.u128),
 505             (sym::NonZeroI8, tcx.types.i8),
 506             (sym::NonZeroI16, tcx.types.i16),
 507             (sym::NonZeroI32, tcx.types.i32),
 508             (sym::NonZeroI64, tcx.types.i64),
 509             (sym::NonZeroI128, tcx.types.i128),
 510         ];
 511
 512         let Some((s, _)) = map
 513             .iter()
 514             .find(|&&(s, t)| self.tcx.is_diagnostic_item(s, adt.did()) && self.can_coerce(expr_ty, t))
 515             else { return false; };
 516
 517         let path = self.tcx.def_path_str(adt.non_enum_variant().def_id);
 518
 519         err.multipart_suggestion(
 520             format!("consider calling `{s}::new`"),
 521             vec![
 522                 (expr.span.shrink_to_lo(), format!("{path}::new(")),
 523                 (expr.span.shrink_to_hi(), format!("){unwrap}")),
 524             ],
 525             Applicability::MaybeIncorrect,
 526         );
 527
 528         true
 529     }
 530
 531     pub fn get_conversion_methods(
 532         &self,
 533         span: Span,
 534         expected: Ty<'tcx>,
 535         checked_ty: Ty<'tcx>,
 536         hir_id: hir::HirId,
 537     ) -> Vec<AssocItem> {
 538         let methods = self.probe_for_return_type(
 539             span,
 540             probe::Mode::MethodCall,
 541             expected,
 542             checked_ty,
 543             hir_id,
 544             |m| {
 545                 self.has_only_self_parameter(m)
 546                     && self
 547                         .tcx
 548                         // This special internal attribute is used to permit
 549                         // "identity-like" conversion methods to be suggested here.
 550                         //
 551                         // FIXME (#46459 and #46460): ideally
 552                         // `std::convert::Into::into` and `std::borrow:ToOwned` would
 553                         // also be `#[rustc_conversion_suggestion]`, if not for
 554                         // method-probing false-positives and -negatives (respectively).
 555                         //
 556                         // FIXME? Other potential candidate methods: `as_ref` and
 557                         // `as_mut`?
 558                         .has_attr(m.def_id, sym::rustc_conversion_suggestion)
 559             },
 560         );
 561
 562         methods
 563     }
 564
 565     /// This function checks whether the method is not static and does not accept other parameters than `self`.
 566     fn has_only_self_parameter(&self, method: &AssocItem) -> bool {
 567         match method.kind {
 568             ty::AssocKind::Fn => {
 569                 method.fn_has_self_parameter
 570                     && self.tcx.fn_sig(method.def_id).inputs().skip_binder().len() == 1
 571             }
 572             _ => false,
 573         }
 574     }
 575
 576     /// Identify some cases where `as_ref()` would be appropriate and suggest it.
 577     ///
 578     /// Given the following code:
 579     /// ```compile_fail,E0308
 580     /// struct Foo;
 581     /// fn takes_ref(_: &Foo) {}
 582     /// let ref opt = Some(Foo);
 583     ///
 584     /// opt.map(|param| takes_ref(param));
 585     /// ```
 586     /// Suggest using `opt.as_ref().map(|param| takes_ref(param));` instead.
 587     ///
 588     /// It only checks for `Option` and `Result` and won't work with
 589     /// ```ignore (illustrative)
 590     /// opt.map(|param| { takes_ref(param) });
 591     /// ```
 592     fn can_use_as_ref(&self, expr: &hir::Expr<'_>) -> Option<(Span, &'static str, String)> {
 593         let hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) = expr.kind else {
 594             return None;
 595         };
 596
 597         let hir::def::Res::Local(local_id) = path.res else {
 598             return None;
 599         };
 600
 601         let local_parent = self.tcx.hir().get_parent_node(local_id);
 602         let Some(Node::Param(hir::Param { hir_id: param_hir_id, .. })) = self.tcx.hir().find(local_parent) else {
 603             return None;
 604         };
 605
 606         let param_parent = self.tcx.hir().get_parent_node(*param_hir_id);
 607         let Some(Node::Expr(hir::Expr {
 608             hir_id: expr_hir_id,
 609             kind: hir::ExprKind::Closure(hir::Closure { fn_decl: closure_fn_decl, .. }),
 610             ..
 611         })) = self.tcx.hir().find(param_parent) else {
 612             return None;
 613         };
 614
 615         let expr_parent = self.tcx.hir().get_parent_node(*expr_hir_id);
 616         let hir = self.tcx.hir().find(expr_parent);
 617         let closure_params_len = closure_fn_decl.inputs.len();
 618         let (
 619             Some(Node::Expr(hir::Expr {
 620                 kind: hir::ExprKind::MethodCall(method_path, receiver, ..),
 621                 ..
 622             })),
 623             1,
 624         ) = (hir, closure_params_len) else {
 625             return None;
 626         };
 627
 628         let self_ty = self.typeck_results.borrow().expr_ty(receiver);
 629         let name = method_path.ident.name;
 630         let is_as_ref_able = match self_ty.peel_refs().kind() {
 631             ty::Adt(def, _) => {
 632                 (self.tcx.is_diagnostic_item(sym::Option, def.did())
 633                     || self.tcx.is_diagnostic_item(sym::Result, def.did()))
 634                     && (name == sym::map || name == sym::and_then)
 635             }
 636             _ => false,
 637         };
 638         match (is_as_ref_able, self.sess().source_map().span_to_snippet(method_path.ident.span)) {
 639             (true, Ok(src)) => {
 640                 let suggestion = format!("as_ref().{}", src);
 641                 Some((method_path.ident.span, "consider using `as_ref` instead", suggestion))
 642             }
 643             _ => None,
 644         }
 645     }
 646
 647     pub(crate) fn maybe_get_struct_pattern_shorthand_field(
 648         &self,
 649         expr: &hir::Expr<'_>,
 650     ) -> Option<Symbol> {
 651         let hir = self.tcx.hir();
 652         let local = match expr {
 653             hir::Expr {
 654                 kind:
 655                     hir::ExprKind::Path(hir::QPath::Resolved(
 656                         None,
 657                         hir::Path {
 658                             res: hir::def::Res::Local(_),
 659                             segments: [hir::PathSegment { ident, .. }],
 660                             ..
 661                         },
 662                     )),
 663                 ..
 664             } => Some(ident),
 665             _ => None,
 666         }?;
 667
 668         match hir.find(hir.get_parent_node(expr.hir_id))? {
 669             Node::ExprField(field) => {
 670                 if field.ident.name == local.name && field.is_shorthand {
 671                     return Some(local.name);
 672                 }
 673             }
 674             _ => {}
 675         }
 676
 677         None
 678     }
 679
 680     /// If the given `HirId` corresponds to a block with a trailing expression, return that expression
 681     pub(crate) fn maybe_get_block_expr(
 682         &self,
 683         expr: &hir::Expr<'tcx>,
 684     ) -> Option<&'tcx hir::Expr<'tcx>> {
 685         match expr {
 686             hir::Expr { kind: hir::ExprKind::Block(block, ..), .. } => block.expr,
 687             _ => None,
 688         }
 689     }
 690
 691     /// Returns whether the given expression is an `else if`.
 692     pub(crate) fn is_else_if_block(&self, expr: &hir::Expr<'_>) -> bool {
 693         if let hir::ExprKind::If(..) = expr.kind {
 694             let parent_id = self.tcx.hir().get_parent_node(expr.hir_id);
 695             if let Some(Node::Expr(hir::Expr {
 696                 kind: hir::ExprKind::If(_, _, Some(else_expr)),
 697                 ..
 698             })) = self.tcx.hir().find(parent_id)
 699             {
 700                 return else_expr.hir_id == expr.hir_id;
 701             }
 702         }
 703         false
 704     }
 705
 706     /// This function is used to determine potential "simple" improvements or users' errors and
 707     /// provide them useful help. For example:
 708     ///
 709     /// ```compile_fail,E0308
 710     /// fn some_fn(s: &str) {}
 711     ///
 712     /// let x = "hey!".to_owned();
 713     /// some_fn(x); // error
 714     /// ```
 715     ///
 716     /// No need to find every potential function which could make a coercion to transform a
 717     /// `String` into a `&str` since a `&` would do the trick!
 718     ///
 719     /// In addition of this check, it also checks between references mutability state. If the
 720     /// expected is mutable but the provided isn't, maybe we could just say "Hey, try with
 721     /// `&mut`!".
 722     pub fn check_ref(
 723         &self,
 724         expr: &hir::Expr<'tcx>,
 725         checked_ty: Ty<'tcx>,
 726         expected: Ty<'tcx>,
 727     ) -> Option<(
 728         Span,
 729         String,
 730         String,
 731         Applicability,
 732         bool, /* verbose */
 733         bool, /* suggest `&` or `&mut` type annotation */
 734     )> {
 735         let sess = self.sess();
 736         let sp = expr.span;
 737
 738         // If the span is from an external macro, there's no suggestion we can make.
 739         if in_external_macro(sess, sp) {
 740             return None;
 741         }
 742
 743         let sm = sess.source_map();
 744
 745         let replace_prefix = |s: &str, old: &str, new: &str| {
 746             s.strip_prefix(old).map(|stripped| new.to_string() + stripped)
 747         };
 748
 749         // `ExprKind::DropTemps` is semantically irrelevant for these suggestions.
 750         let expr = expr.peel_drop_temps();
 751
 752         match (&expr.kind, expected.kind(), checked_ty.kind()) {
 753             (_, &ty::Ref(_, exp, _), &ty::Ref(_, check, _)) => match (exp.kind(), check.kind()) {
 754                 (&ty::Str, &ty::Array(arr, _) | &ty::Slice(arr)) if arr == self.tcx.types.u8 => {
 755                     if let hir::ExprKind::Lit(_) = expr.kind
 756                         && let Ok(src) = sm.span_to_snippet(sp)
 757                         && replace_prefix(&src, "b\"", "\"").is_some()
 758                     {
 759                                 let pos = sp.lo() + BytePos(1);
 760                                 return Some((
 761                                     sp.with_hi(pos),
 762                                     "consider removing the leading `b`".to_string(),
 763                                     String::new(),
 764                                     Applicability::MachineApplicable,
 765                                     true,
 766                                     false,
 767                                 ));
 768                             }
 769                         }
 770                 (&ty::Array(arr, _) | &ty::Slice(arr), &ty::Str) if arr == self.tcx.types.u8 => {
 771                     if let hir::ExprKind::Lit(_) = expr.kind
 772                         && let Ok(src) = sm.span_to_snippet(sp)
 773                         && replace_prefix(&src, "\"", "b\"").is_some()
 774                     {
 775                                 return Some((
 776                                     sp.shrink_to_lo(),
 777                                     "consider adding a leading `b`".to_string(),
 778                                     "b".to_string(),
 779                                     Applicability::MachineApplicable,
 780                                     true,
 781                                     false,
 782                                 ));
 783                     }
 784                 }
 785                 _ => {}
 786             },
 787             (_, &ty::Ref(_, _, mutability), _) => {
 788                 // Check if it can work when put into a ref. For example:
 789                 //
 790                 // ```
 791                 // fn bar(x: &mut i32) {}
 792                 //
 793                 // let x = 0u32;
 794                 // bar(&x); // error, expected &mut
 795                 // ```
 796                 let ref_ty = match mutability {
 797                     hir::Mutability::Mut => {
 798                         self.tcx.mk_mut_ref(self.tcx.mk_region(ty::ReStatic), checked_ty)
 799                     }
 800                     hir::Mutability::Not => {
 801                         self.tcx.mk_imm_ref(self.tcx.mk_region(ty::ReStatic), checked_ty)
 802                     }
 803                 };
 804                 if self.can_coerce(ref_ty, expected) {
 805                     let mut sugg_sp = sp;
 806                     if let hir::ExprKind::MethodCall(ref segment, receiver, args, _) = expr.kind {
 807                         let clone_trait =
 808                             self.tcx.require_lang_item(LangItem::Clone, Some(segment.ident.span));
 809                         if args.is_empty()
 810                             && self.typeck_results.borrow().type_dependent_def_id(expr.hir_id).map(
 811                                 |did| {
 812                                     let ai = self.tcx.associated_item(did);
 813                                     ai.trait_container(self.tcx) == Some(clone_trait)
 814                                 },
 815                             ) == Some(true)
 816                             && segment.ident.name == sym::clone
 817                         {
 818                             // If this expression had a clone call when suggesting borrowing
 819                             // we want to suggest removing it because it'd now be unnecessary.
 820                             sugg_sp = receiver.span;
 821                         }
 822                     }
 823                     if let Ok(src) = sm.span_to_snippet(sugg_sp) {
 824                         let needs_parens = match expr.kind {
 825                             // parenthesize if needed (Issue #46756)
 826                             hir::ExprKind::Cast(_, _) | hir::ExprKind::Binary(_, _, _) => true,
 827                             // parenthesize borrows of range literals (Issue #54505)
 828                             _ if is_range_literal(expr) => true,
 829                             _ => false,
 830                         };
 831
 832                         if let Some(sugg) = self.can_use_as_ref(expr) {
 833                             return Some((
 834                                 sugg.0,
 835                                 sugg.1.to_string(),
 836                                 sugg.2,
 837                                 Applicability::MachineApplicable,
 838                                 false,
 839                                 false,
 840                             ));
 841                         }
 842
 843                         let prefix = match self.maybe_get_struct_pattern_shorthand_field(expr) {
 844                             Some(ident) => format!("{ident}: "),
 845                             None => String::new(),
 846                         };
 847
 848                         if let Some(hir::Node::Expr(hir::Expr {
 849                             kind: hir::ExprKind::Assign(..),
 850                             ..
 851                         })) = self.tcx.hir().find(self.tcx.hir().get_parent_node(expr.hir_id))
 852                         {
 853                             if mutability == hir::Mutability::Mut {
 854                                 // Suppressing this diagnostic, we'll properly print it in `check_expr_assign`
 855                                 return None;
 856                             }
 857                         }
 858
 859                         let sugg_expr = if needs_parens { format!("({src})") } else { src };
 860                         return Some(match mutability {
 861                             hir::Mutability::Mut => (
 862                                 sp,
 863                                 "consider mutably borrowing here".to_string(),
 864                                 format!("{prefix}&mut {sugg_expr}"),
 865                                 Applicability::MachineApplicable,
 866                                 false,
 867                                 false,
 868                             ),
 869                             hir::Mutability::Not => (
 870                                 sp,
 871                                 "consider borrowing here".to_string(),
 872                                 format!("{prefix}&{sugg_expr}"),
 873                                 Applicability::MachineApplicable,
 874                                 false,
 875                                 false,
 876                             ),
 877                         });
 878                     }
 879                 }
 880             }
 881             (
 882                 hir::ExprKind::AddrOf(hir::BorrowKind::Ref, _, ref expr),
 883                 _,
 884                 &ty::Ref(_, checked, _),
 885             ) if self.can_sub(self.param_env, checked, expected).is_ok() => {
 886                 // We have `&T`, check if what was expected was `T`. If so,
 887                 // we may want to suggest removing a `&`.
 888                 if sm.is_imported(expr.span) {
 889                     // Go through the spans from which this span was expanded,
 890                     // and find the one that's pointing inside `sp`.
 891                     //
 892                     // E.g. for `&format!("")`, where we want the span to the
 893                     // `format!()` invocation instead of its expansion.
 894                     if let Some(call_span) =
 895                         iter::successors(Some(expr.span), |s| s.parent_callsite())
 896                             .find(|&s| sp.contains(s))
 897                         && sm.is_span_accessible(call_span)
 898                     {
 899                         return Some((
 900                             sp.with_hi(call_span.lo()),
 901                             "consider removing the borrow".to_string(),
 902                             String::new(),
 903                             Applicability::MachineApplicable,
 904                             true,
 905                             true
 906                         ));
 907                     }
 908                     return None;
 909                 }
 910                 if sp.contains(expr.span)
 911                     && sm.is_span_accessible(expr.span)
 912                 {
 913                     return Some((
 914                         sp.with_hi(expr.span.lo()),
 915                         "consider removing the borrow".to_string(),
 916                         String::new(),
 917                         Applicability::MachineApplicable,
 918                         true,
 919                         true,
 920                     ));
 921                 }
 922             }
 923             (
 924                 _,
 925                 &ty::RawPtr(TypeAndMut { ty: ty_b, mutbl: mutbl_b }),
 926                 &ty::Ref(_, ty_a, mutbl_a),
 927             ) => {
 928                 if let Some(steps) = self.deref_steps(ty_a, ty_b)
 929                     // Only suggest valid if dereferencing needed.
 930                     && steps > 0
 931                     // The pointer type implements `Copy` trait so the suggestion is always valid.
 932                     && let Ok(src) = sm.span_to_snippet(sp)
 933                 {
 934                     let derefs = "*".repeat(steps);
 935                     if let Some((span, src, applicability)) = match mutbl_b {
 936                         hir::Mutability::Mut => {
 937                             let new_prefix = "&mut ".to_owned() + &derefs;
 938                             match mutbl_a {
 939                                 hir::Mutability::Mut => {
 940                                     replace_prefix(&src, "&mut ", &new_prefix).map(|_| {
 941                                         let pos = sp.lo() + BytePos(5);
 942                                         let sp = sp.with_lo(pos).with_hi(pos);
 943                                         (sp, derefs, Applicability::MachineApplicable)
 944                                     })
 945                                 }
 946                                 hir::Mutability::Not => {
 947                                     replace_prefix(&src, "&", &new_prefix).map(|_| {
 948                                         let pos = sp.lo() + BytePos(1);
 949                                         let sp = sp.with_lo(pos).with_hi(pos);
 950                                         (
 951                                             sp,
 952                                             format!("mut {derefs}"),
 953                                             Applicability::Unspecified,
 954                                         )
 955                                     })
 956                                 }
 957                             }
 958                         }
 959                         hir::Mutability::Not => {
 960                             let new_prefix = "&".to_owned() + &derefs;
 961                             match mutbl_a {
 962                                 hir::Mutability::Mut => {
 963                                     replace_prefix(&src, "&mut ", &new_prefix).map(|_| {
 964                                         let lo = sp.lo() + BytePos(1);
 965                                         let hi = sp.lo() + BytePos(5);
 966                                         let sp = sp.with_lo(lo).with_hi(hi);
 967                                         (sp, derefs, Applicability::MachineApplicable)
 968                                     })
 969                                 }
 970                                 hir::Mutability::Not => {
 971                                     replace_prefix(&src, "&", &new_prefix).map(|_| {
 972                                         let pos = sp.lo() + BytePos(1);
 973                                         let sp = sp.with_lo(pos).with_hi(pos);
 974                                         (sp, derefs, Applicability::MachineApplicable)
 975                                     })
 976                                 }
 977                             }
 978                         }
 979                     } {
 980                         return Some((
 981                             span,
 982                             "consider dereferencing".to_string(),
 983                             src,
 984                             applicability,
 985                             true,
 986                             false,
 987                         ));
 988                     }
 989                 }
 990             }
 991             _ if sp == expr.span => {
 992                 if let Some(mut steps) = self.deref_steps(checked_ty, expected) {
 993                     let mut expr = expr.peel_blocks();
 994                     let mut prefix_span = expr.span.shrink_to_lo();
 995                     let mut remove = String::new();
 996
 997                     // Try peeling off any existing `&` and `&mut` to reach our target type
 998                     while steps > 0 {
 999                         if let hir::ExprKind::AddrOf(_, mutbl, inner) = expr.kind {
1000                             // If the expression has `&`, removing it would fix the error
1001                             prefix_span = prefix_span.with_hi(inner.span.lo());
1002                             expr = inner;
1003                             remove += match mutbl {
1004                                 hir::Mutability::Not => "&",
1005                                 hir::Mutability::Mut => "&mut ",
1006                             };
1007                             steps -= 1;
1008                         } else {
1009                             break;
1010                         }
1011                     }
1012                     // If we've reached our target type with just removing `&`, then just print now.
1013                     if steps == 0 {
1014                         return Some((
1015                             prefix_span,
1016                             format!("consider removing the `{}`", remove.trim()),
1017                             String::new(),
1018                             // Do not remove `&&` to get to bool, because it might be something like
1019                             // { a } && b, which we have a separate fixup suggestion that is more
1020                             // likely correct...
1021                             if remove.trim() == "&&" && expected == self.tcx.types.bool {
1022                                 Applicability::MaybeIncorrect
1023                             } else {
1024                                 Applicability::MachineApplicable
1025                             },
1026                             true,
1027                             false,
1028                         ));
1029                     }
1030
1031                     // For this suggestion to make sense, the type would need to be `Copy`,
1032                     // or we have to be moving out of a `Box<T>`
1033                     if self.type_is_copy_modulo_regions(self.param_env, expected, sp)
1034                         // FIXME(compiler-errors): We can actually do this if the checked_ty is
1035                         // `steps` layers of boxes, not just one, but this is easier and most likely.
1036                         || (checked_ty.is_box() && steps == 1)
1037                     {
1038                         let deref_kind = if checked_ty.is_box() {
1039                             "unboxing the value"
1040                         } else if checked_ty.is_region_ptr() {
1041                             "dereferencing the borrow"
1042                         } else {
1043                             "dereferencing the type"
1044                         };
1045
1046                         // Suggest removing `&` if we have removed any, otherwise suggest just
1047                         // dereferencing the remaining number of steps.
1048                         let message = if remove.is_empty() {
1049                             format!("consider {deref_kind}")
1050                         } else {
1051                             format!(
1052                                 "consider removing the `{}` and {} instead",
1053                                 remove.trim(),
1054                                 deref_kind
1055                             )
1056                         };
1057
1058                         let prefix = match self.maybe_get_struct_pattern_shorthand_field(expr) {
1059                             Some(ident) => format!("{ident}: "),
1060                             None => String::new(),
1061                         };
1062
1063                         let (span, suggestion) = if self.is_else_if_block(expr) {
1064                             // Don't suggest nonsense like `else *if`
1065                             return None;
1066                         } else if let Some(expr) = self.maybe_get_block_expr(expr) {
1067                             // prefix should be empty here..
1068                             (expr.span.shrink_to_lo(), "*".to_string())
1069                         } else {
1070                             (prefix_span, format!("{}{}", prefix, "*".repeat(steps)))
1071                         };
1072
1073                         return Some((
1074                             span,
1075                             message,
1076                             suggestion,
1077                             Applicability::MachineApplicable,
1078                             true,
1079                             false,
1080                         ));
1081                     }
1082                 }
1083             }
1084             _ => {}
1085         }
1086         None
1087     }
1088
1089     pub fn check_for_cast(
1090         &self,
1091         err: &mut Diagnostic,
1092         expr: &hir::Expr<'_>,
1093         checked_ty: Ty<'tcx>,
1094         expected_ty: Ty<'tcx>,
1095         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
1096     ) -> bool {
1097         if self.tcx.sess.source_map().is_imported(expr.span) {
1098             // Ignore if span is from within a macro.
1099             return false;
1100         }
1101
1102         let Ok(src) = self.tcx.sess.source_map().span_to_snippet(expr.span) else {
1103             return false;
1104         };
1105
1106         // If casting this expression to a given numeric type would be appropriate in case of a type
1107         // mismatch.
1108         //
1109         // We want to minimize the amount of casting operations that are suggested, as it can be a
1110         // lossy operation with potentially bad side effects, so we only suggest when encountering
1111         // an expression that indicates that the original type couldn't be directly changed.
1112         //
1113         // For now, don't suggest casting with `as`.
1114         let can_cast = false;
1115
1116         let mut sugg = vec![];
1117
1118         if let Some(hir::Node::ExprField(field)) =
1119             self.tcx.hir().find(self.tcx.hir().get_parent_node(expr.hir_id))
1120         {
1121             // `expr` is a literal field for a struct, only suggest if appropriate
1122             if field.is_shorthand {
1123                 // This is a field literal
1124                 sugg.push((field.ident.span.shrink_to_lo(), format!("{}: ", field.ident)));
1125             } else {
1126                 // Likely a field was meant, but this field wasn't found. Do not suggest anything.
1127                 return false;
1128             }
1129         };
1130
1131         if let hir::ExprKind::Call(path, args) = &expr.kind
1132             && let (hir::ExprKind::Path(hir::QPath::TypeRelative(base_ty, path_segment)), 1) =
1133                 (&path.kind, args.len())
1134             // `expr` is a conversion like `u32::from(val)`, do not suggest anything (#63697).
1135             && let (hir::TyKind::Path(hir::QPath::Resolved(None, base_ty_path)), sym::from) =
1136                 (&base_ty.kind, path_segment.ident.name)
1137         {
1138             if let Some(ident) = &base_ty_path.segments.iter().map(|s| s.ident).next() {
1139                 match ident.name {
1140                     sym::i128
1141                     | sym::i64
1142                     | sym::i32
1143                     | sym::i16
1144                     | sym::i8
1145                     | sym::u128
1146                     | sym::u64
1147                     | sym::u32
1148                     | sym::u16
1149                     | sym::u8
1150                     | sym::isize
1151                     | sym::usize
1152                         if base_ty_path.segments.len() == 1 =>
1153                     {
1154                         return false;
1155                     }
1156                     _ => {}
1157                 }
1158             }
1159         }
1160
1161         let msg = format!(
1162             "you can convert {} `{}` to {} `{}`",
1163             checked_ty.kind().article(),
1164             checked_ty,
1165             expected_ty.kind().article(),
1166             expected_ty,
1167         );
1168         let cast_msg = format!(
1169             "you can cast {} `{}` to {} `{}`",
1170             checked_ty.kind().article(),
1171             checked_ty,
1172             expected_ty.kind().article(),
1173             expected_ty,
1174         );
1175         let lit_msg = format!(
1176             "change the type of the numeric literal from `{checked_ty}` to `{expected_ty}`",
1177         );
1178
1179         let close_paren = if expr.precedence().order() < PREC_POSTFIX {
1180             sugg.push((expr.span.shrink_to_lo(), "(".to_string()));
1181             ")"
1182         } else {
1183             ""
1184         };
1185
1186         let mut cast_suggestion = sugg.clone();
1187         cast_suggestion.push((expr.span.shrink_to_hi(), format!("{close_paren} as {expected_ty}")));
1188         let mut into_suggestion = sugg.clone();
1189         into_suggestion.push((expr.span.shrink_to_hi(), format!("{close_paren}.into()")));
1190         let mut suffix_suggestion = sugg.clone();
1191         suffix_suggestion.push((
1192             if matches!(
1193                 (&expected_ty.kind(), &checked_ty.kind()),
1194                 (ty::Int(_) | ty::Uint(_), ty::Float(_))
1195             ) {
1196                 // Remove fractional part from literal, for example `42.0f32` into `42`
1197                 let src = src.trim_end_matches(&checked_ty.to_string());
1198                 let len = src.split('.').next().unwrap().len();
1199                 expr.span.with_lo(expr.span.lo() + BytePos(len as u32))
1200             } else {
1201                 let len = src.trim_end_matches(&checked_ty.to_string()).len();
1202                 expr.span.with_lo(expr.span.lo() + BytePos(len as u32))
1203             },
1204             if expr.precedence().order() < PREC_POSTFIX {
1205                 // Readd `)`
1206                 format!("{expected_ty})")
1207             } else {
1208                 expected_ty.to_string()
1209             },
1210         ));
1211         let literal_is_ty_suffixed = |expr: &hir::Expr<'_>| {
1212             if let hir::ExprKind::Lit(lit) = &expr.kind { lit.node.is_suffixed() } else { false }
1213         };
1214         let is_negative_int =
1215             |expr: &hir::Expr<'_>| matches!(expr.kind, hir::ExprKind::Unary(hir::UnOp::Neg, ..));
1216         let is_uint = |ty: Ty<'_>| matches!(ty.kind(), ty::Uint(..));
1217
1218         let in_const_context = self.tcx.hir().is_inside_const_context(expr.hir_id);
1219
1220         let suggest_fallible_into_or_lhs_from =
1221             |err: &mut Diagnostic, exp_to_found_is_fallible: bool| {
1222                 // If we know the expression the expected type is derived from, we might be able
1223                 // to suggest a widening conversion rather than a narrowing one (which may
1224                 // panic). For example, given x: u8 and y: u32, if we know the span of "x",
1225                 //   x > y
1226                 // can be given the suggestion "u32::from(x) > y" rather than
1227                 // "x > y.try_into().unwrap()".
1228                 let lhs_expr_and_src = expected_ty_expr.and_then(|expr| {
1229                     self.tcx
1230                         .sess
1231                         .source_map()
1232                         .span_to_snippet(expr.span)
1233                         .ok()
1234                         .map(|src| (expr, src))
1235                 });
1236                 let (msg, suggestion) = if let (Some((lhs_expr, lhs_src)), false) =
1237                     (lhs_expr_and_src, exp_to_found_is_fallible)
1238                 {
1239                     let msg = format!(
1240                         "you can convert `{lhs_src}` from `{expected_ty}` to `{checked_ty}`, matching the type of `{src}`",
1241                     );
1242                     let suggestion = vec![
1243                         (lhs_expr.span.shrink_to_lo(), format!("{checked_ty}::from(")),
1244                         (lhs_expr.span.shrink_to_hi(), ")".to_string()),
1245                     ];
1246                     (msg, suggestion)
1247                 } else {
1248                     let msg = format!("{msg} and panic if the converted value doesn't fit");
1249                     let mut suggestion = sugg.clone();
1250                     suggestion.push((
1251                         expr.span.shrink_to_hi(),
1252                         format!("{close_paren}.try_into().unwrap()"),
1253                     ));
1254                     (msg, suggestion)
1255                 };
1256                 err.multipart_suggestion_verbose(
1257                     &msg,
1258                     suggestion,
1259                     Applicability::MachineApplicable,
1260                 );
1261             };
1262
1263         let suggest_to_change_suffix_or_into =
1264             |err: &mut Diagnostic,
1265              found_to_exp_is_fallible: bool,
1266              exp_to_found_is_fallible: bool| {
1267                 let exp_is_lhs =
1268                     expected_ty_expr.map(|e| self.tcx.hir().is_lhs(e.hir_id)).unwrap_or(false);
1269
1270                 if exp_is_lhs {
1271                     return;
1272                 }
1273
1274                 let always_fallible = found_to_exp_is_fallible
1275                     && (exp_to_found_is_fallible || expected_ty_expr.is_none());
1276                 let msg = if literal_is_ty_suffixed(expr) {
1277                     &lit_msg
1278                 } else if always_fallible && (is_negative_int(expr) && is_uint(expected_ty)) {
1279                     // We now know that converting either the lhs or rhs is fallible. Before we
1280                     // suggest a fallible conversion, check if the value can never fit in the
1281                     // expected type.
1282                     let msg = format!("`{src}` cannot fit into type `{expected_ty}`");
1283                     err.note(&msg);
1284                     return;
1285                 } else if in_const_context {
1286                     // Do not recommend `into` or `try_into` in const contexts.
1287                     return;
1288                 } else if found_to_exp_is_fallible {
1289                     return suggest_fallible_into_or_lhs_from(err, exp_to_found_is_fallible);
1290                 } else {
1291                     &msg
1292                 };
1293                 let suggestion = if literal_is_ty_suffixed(expr) {
1294                     suffix_suggestion.clone()
1295                 } else {
1296                     into_suggestion.clone()
1297                 };
1298                 err.multipart_suggestion_verbose(msg, suggestion, Applicability::MachineApplicable);
1299             };
1300
1301         match (&expected_ty.kind(), &checked_ty.kind()) {
1302             (&ty::Int(ref exp), &ty::Int(ref found)) => {
1303                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
1304                 {
1305                     (Some(exp), Some(found)) if exp < found => (true, false),
1306                     (Some(exp), Some(found)) if exp > found => (false, true),
1307                     (None, Some(8 | 16)) => (false, true),
1308                     (Some(8 | 16), None) => (true, false),
1309                     (None, _) | (_, None) => (true, true),
1310                     _ => (false, false),
1311                 };
1312                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
1313                 true
1314             }
1315             (&ty::Uint(ref exp), &ty::Uint(ref found)) => {
1316                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
1317                 {
1318                     (Some(exp), Some(found)) if exp < found => (true, false),
1319                     (Some(exp), Some(found)) if exp > found => (false, true),
1320                     (None, Some(8 | 16)) => (false, true),
1321                     (Some(8 | 16), None) => (true, false),
1322                     (None, _) | (_, None) => (true, true),
1323                     _ => (false, false),
1324                 };
1325                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
1326                 true
1327             }
1328             (&ty::Int(exp), &ty::Uint(found)) => {
1329                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
1330                 {
1331                     (Some(exp), Some(found)) if found < exp => (false, true),
1332                     (None, Some(8)) => (false, true),
1333                     _ => (true, true),
1334                 };
1335                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
1336                 true
1337             }
1338             (&ty::Uint(exp), &ty::Int(found)) => {
1339                 let (f2e_is_fallible, e2f_is_fallible) = match (exp.bit_width(), found.bit_width())
1340                 {
1341                     (Some(exp), Some(found)) if found > exp => (true, false),
1342                     (Some(8), None) => (true, false),
1343                     _ => (true, true),
1344                 };
1345                 suggest_to_change_suffix_or_into(err, f2e_is_fallible, e2f_is_fallible);
1346                 true
1347             }
1348             (&ty::Float(ref exp), &ty::Float(ref found)) => {
1349                 if found.bit_width() < exp.bit_width() {
1350                     suggest_to_change_suffix_or_into(err, false, true);
1351                 } else if literal_is_ty_suffixed(expr) {
1352                     err.multipart_suggestion_verbose(
1353                         &lit_msg,
1354                         suffix_suggestion,
1355                         Applicability::MachineApplicable,
1356                     );
1357                 } else if can_cast {
1358                     // Missing try_into implementation for `f64` to `f32`
1359                     err.multipart_suggestion_verbose(
1360                         &format!("{cast_msg}, producing the closest possible value"),
1361                         cast_suggestion,
1362                         Applicability::MaybeIncorrect, // lossy conversion
1363                     );
1364                 }
1365                 true
1366             }
1367             (&ty::Uint(_) | &ty::Int(_), &ty::Float(_)) => {
1368                 if literal_is_ty_suffixed(expr) {
1369                     err.multipart_suggestion_verbose(
1370                         &lit_msg,
1371                         suffix_suggestion,
1372                         Applicability::MachineApplicable,
1373                     );
1374                 } else if can_cast {
1375                     // Missing try_into implementation for `{float}` to `{integer}`
1376                     err.multipart_suggestion_verbose(
1377                         &format!("{msg}, rounding the float towards zero"),
1378                         cast_suggestion,
1379                         Applicability::MaybeIncorrect, // lossy conversion
1380                     );
1381                 }
1382                 true
1383             }
1384             (&ty::Float(ref exp), &ty::Uint(ref found)) => {
1385                 // if `found` is `None` (meaning found is `usize`), don't suggest `.into()`
1386                 if exp.bit_width() > found.bit_width().unwrap_or(256) {
1387                     err.multipart_suggestion_verbose(
1388                         &format!(
1389                             "{msg}, producing the floating point representation of the integer",
1390                         ),
1391                         into_suggestion,
1392                         Applicability::MachineApplicable,
1393                     );
1394                 } else if literal_is_ty_suffixed(expr) {
1395                     err.multipart_suggestion_verbose(
1396                         &lit_msg,
1397                         suffix_suggestion,
1398                         Applicability::MachineApplicable,
1399                     );
1400                 } else {
1401                     // Missing try_into implementation for `{integer}` to `{float}`
1402                     err.multipart_suggestion_verbose(
1403                         &format!(
1404                             "{cast_msg}, producing the floating point representation of the integer, \
1405                                  rounded if necessary",
1406                         ),
1407                         cast_suggestion,
1408                         Applicability::MaybeIncorrect, // lossy conversion
1409                     );
1410                 }
1411                 true
1412             }
1413             (&ty::Float(ref exp), &ty::Int(ref found)) => {
1414                 // if `found` is `None` (meaning found is `isize`), don't suggest `.into()`
1415                 if exp.bit_width() > found.bit_width().unwrap_or(256) {
1416                     err.multipart_suggestion_verbose(
1417                         &format!(
1418                             "{}, producing the floating point representation of the integer",
1419                             &msg,
1420                         ),
1421                         into_suggestion,
1422                         Applicability::MachineApplicable,
1423                     );
1424                 } else if literal_is_ty_suffixed(expr) {
1425                     err.multipart_suggestion_verbose(
1426                         &lit_msg,
1427                         suffix_suggestion,
1428                         Applicability::MachineApplicable,
1429                     );
1430                 } else {
1431                     // Missing try_into implementation for `{integer}` to `{float}`
1432                     err.multipart_suggestion_verbose(
1433                         &format!(
1434                             "{}, producing the floating point representation of the integer, \
1435                                 rounded if necessary",
1436                             &msg,
1437                         ),
1438                         cast_suggestion,
1439                         Applicability::MaybeIncorrect, // lossy conversion
1440                     );
1441                 }
1442                 true
1443             }
1444             (
1445                 &ty::Uint(ty::UintTy::U32 | ty::UintTy::U64 | ty::UintTy::U128)
1446                 | &ty::Int(ty::IntTy::I32 | ty::IntTy::I64 | ty::IntTy::I128),
1447                 &ty::Char,
1448             ) => {
1449                 err.multipart_suggestion_verbose(
1450                     &format!("{cast_msg}, since a `char` always occupies 4 bytes"),
1451                     cast_suggestion,
1452                     Applicability::MachineApplicable,
1453                 );
1454                 true
1455             }
1456             _ => false,
1457         }
1458     }
1459 }